This invention relates to an antenna and more particularly to an antenna for use with a transponder forming part of an electronic radio frequency identification system comprising an interrogator or reader and a plurality of transponders.
Known electronic identification systems of the aforementioned kind comprise an interrogator comprising a transmitter for transmitting an interrogation signal to the transponders and a receiver for receiving a response signal from the transponders. A microprocessor in the interrogator identifies a particular transponder from data in the response signal received from that transponder. Each transponder comprises an antenna and a detector circuit for receiving and collecting power from the interrogation signal, to present a high enough voltage to power a modulator and logic circuitry of the transponder.
The antennas that are being used in the known transponders are divided into two main groups, namely conventional linear half-wavelength resonant dipole antennas and loop antennas. The main disadvantage of the half-wave dipole antenna is that it is too long for many applications. The main disadvantage of the loop antenna is that its radiation and response pattern is too dependent on the proximity of other objects. In a case where a plurality of transponders are randomly orientated, this dependence may result in some of the transponders not receiving the interrogation signal from the interrogator.
Accordingly it is an object of the present invention to provide an antenna with which the applicant believes the aforementioned disadvantages may at least be alleviated.
According to the invention there is provided a dipole antenna wherein a first pole comprises an electrically conductive loop and a second pole comprises an electrically conductive appendage extending outwardly from the loop.
The loop may have any suitable shape or configuration such as a circular shape, oval shape, square shape, rectangular shape etc and the circumference of the loop is preferably substantially equal to one half wavelength of an operational frequency.
The loop is preferably rectangular in shape.
The appendage has a length which is preferably at least 0.15 times the aforementioned wavelength and may comprise a linear strip having a length of at least 0.15 times the aforementioned wavelength.
In other embodiments the strip may be shorter and be a link terminating in a solid electrically conductive flat body or pad.
In a preferred embodiment an outer profile of the loop and an outer profile of the pad are the same.
The antenna may be symmetrical about a center axis extending through the loop, the appendage and the pad. The antenna may also be symmetrical about an axis extending perpendicularly to the centre axis.
A feedpoint for the antenna may be provided at an interruption in the loop.
The antenna may have an antenna impedance at the feedpoint which is dependent on a position selected on the loop for the feedpoint.
The loop may comprise impedance matching means.
Also included within the scope of the present invention is a transponder of a radio frequency electronic identification system which transponder comprises an antenna as hereinbefore defined. An electronic identification system comprising such transponders is also included within the scope of the present invention.